For detecting a bed occupancy state, such as, e.g., a state in which a subject (e.g., a patient, an aged person, a care recipient, a healthy person, or a baby) on a bed platform is getting out of a bed, a state in which a subject is getting onto a bed, or a position of the center of gravity of a subject, the following method is known for example.
First to fourth load detectors are disposed under the head side right leg portion, the head side left leg portion, the foot side right leg portion, and the foot side left leg portion of a bed, respectively. These four load detectors detect the loads applied to the head side right portion of the bed platform, the head side left portion thereof, the foot side right portion thereof, and the foot side left portion thereof. Using the outputted loads (load values) from these four load detectors, the gravity center of the subject on the bed platform is calculated. The coordinate (GX, GY) of the center of gravity of the object can be calculated from the following formulas (101) and (102).GX=(W1+W2−W3−W4)×(1/WT)×(BX/2)  Formula (101)GY=(W1+W3−W2−W4)×(1/WT)×(BY/2)  Formula (102)
where,
W1: output load (load value) from the first load detector
W2: output load (load value) from the second load detector
W3: output load (load value) from the third load detector
W4: output load (load value) from the fourth load detector
BX: distance between the leg portions of the bed in the length direction (i.e., in the X-axis direction)
BY: distance between the leg portions of the bed in the width direction (i.e., in the Y-axis)WT=W1+W2+W3+W4
Based on the position of the center of gravity calculated by these gravity center position formulas (101) and (102), the moving direction, the moving speed, etc., of the subject on the bed platform are calculated. From the calculated results, the bed occupancy state of the subject will be detected (see, e.g., Patent Documents 1 and 2).
In the meantime, if values corresponding to lying postures of a subject on a bed platform is detected, it is useful on the following points. That is, for example, the detection enables prediction of the subject's getting out of the bed or prediction of the possibility of the subject's falling down from the bed platform. Therefore, it is important to detect values corresponding to the lying postures of a subject.    Patent Document 1: Japanese Patent Publication No. 3,322,632    Patent Document 2: Japanese Patent Publication No. 3,093,745